Spirit Airlines said on Monday it had filed for bankruptcy protection, after the pioneer of no-frills travel in the US struggled with a long run of quarterly losses, failed merger attempts and looming debt maturities.

Spirit, the biggest US budget airline, had been losing money despite strong travel demand as it struggled with higher operating costs.

Spirit's troubles deepened after the collapse of its US$3.8 billion planned merger with JetBlue Airways in January and the impact of RTX's Pratt & Whitney Geared Turbofan (GTF) engines snag that grounded many of its aircraft.

The airline listed its estimated assets and liabilities in the range of US$1 billion to US$10 billion each, according to a court filing on Monday.

Spirit has entered into an agreement with its bondholders that is expected to reduce total debt and provide increased financial flexibility.

The airline, as part of the prearranged Chapter 11 bankruptcy protection, has received commitment for a US$350 million equity investment from existing bondholders.

Existing bondholders will also provide US$300 million in debtor-in-possession (DIP) financing, which, together with available cash, is expected to support the airline through the Chapter 11 process.

The carrier said it expected to continue its flight operations through the proceedings and customers can book and fly without interruption.

Spirit expects to be delisted from the New York Stock Exchange in the near term.

The company said it expected to emerge from the Chapter 11 process in the first quarter of 2025.

Spirit's shares, halted for trading on Monday, have plunged more than 90 percent this year.

The company started out as a long-haul trucking company in 1964 before shifting to aviation around 1983. It offered leisure packages to popular destinations under the name Charter One Airlines and rebranded to Spirit in 1992. (Reuters/AP)